PMX40 RF Power Meter – INSTRUCTION MANUAL
5-1
5
Application Notes
This section provides supplementary material to enhance your knowledge of Model PMX40 operation,
advanced features, and measurement accuracy. Topics covered in this section include pulse
measurement fundamentals, automatic measurement principles, and an analysis of measurement
accuracy.
5.1
Introduction to Pulse Measurements
5.1.1
Measurement Fundamentals
The following is a brief review of power measurement fundamentals.
Unmodulated Carrier Power
. The average power of an unmodulated carrier consisting of a continuous,
constant amplitude sinewave signal is also termed continuous wave (CW) power. For a known value of
load impedance R, and applied voltage V
rms
, the average power is:
P = V
rms
²/R watts
Power meters designed to measure CW power can use thermoelectric-based sensors which respond to
the heating effect of the signal or diode detectors which respond to the voltage of the signal. With
careful calibration accurate measurements can be obtained over a wide range of input power levels.
Modulated Carrier Power
. The average power of a modulated carrier which has varying amplitude can
be measured accurately by a CW type power meter with a thermoelectric detector, but the lack of
sensitivity will limit the range. Diode detectors can be used at low power, square-law response levels. At
higher power levels the diode responds in a more linear manner and significant error results.
Pulse Power
. Pulse power refers to power measured during the on time of pulsed RF signals (Figure 5-1).
Traditionally, these signals have been measured in two steps: (1) thermoelectric sensors measure the
average signal power, (2) the reading is then divided by the duty cycle to obtain pulse power, P
pulse
:
P
pulse
= Average Power/ Duty Cycle, where Duty Cycle = Pulse Width/Pulse Period
Pulse power provides useful results when applied to rectangular pulses, but is inaccurate for pulse
shapes that include distortions, such as overshoot or droop (Figure 5-2).
